A general synthetic method involving the intramolecular dipolar cycloaddition reactions of nitrilium betaines which should be applicable to the synthesis of numerous fused heterocyclic rings will be explored. The scope and mechanism of the 1,1-cycloaddition reaction of nitrile ylides, nitrile imines and nitrile oxides will be studied. The regio-and stereospecificity of the intramolecular 1,1-cycloaddition with electron rich and electron deficient olefins as well as with hetero multiple pi-bonds will be evaluated with respect to the synthesis of important biologically active compounds. The development of general synthetic routes leading to unusual tricyclic systems via the reaction of cyclopropene derivatives will also be examined.